In the forestry industry wheeled and tracked feller buncher machines may be utilized to harvest standing trees. Saw heads may be mounted to the feller buncher machines, for example to a boom arm, which have one or more large saw discs with peripheral teeth. The saw head may be driven by the hydraulic system of the machine such that the saw disc is rotated. The rotating saw may thereby cut into tree trunks to fell the standing trees. The saw head may include a large grapple or other clamping arrangement, which may be secured around the tree trunk so that after the tree is felled it may be carried to a transport machine or stacked for later pick-up and transport by a forwarder or the like.
During operation, the saw disc may be rotated with large angular velocities to facilitate efficient cutting of the selected trees. It is common to leverage kinetic inertia to a large degree during a felling operation, such that much of the cutting force applied by the saw head is due to the relatively large inertia of the saw disc. In such a case, the hydraulic system may be configured primarily to initiate start-up rotation of the saw disc, by driving the saw disc to overcome the static inertia of the saw disc. In particular, the hydraulic system is required to drive the saw disc during periods of recovery (e.g., after the saw disc is slowed from a prior felling operation) to bring it up to speed for the current felling operation. Typically, it is desired that the recovery time be minimized so that the machine may perform consecutive felling operations rapidly. Once a saw disc is spinning at operating speeds, the hydraulic system may also be utilized to maintain the saw speed at appropriate levels.
It is known to control the hydraulic system of the machine in order to supply the necessary power to meet the demands of the saw head. Available power in part is dependent on the state of the engine and the demands of the other hydraulic systems of the machine. Some control schemes regulate hydraulic power to the saw head according to engine speed. Other schemes seek to prioritize saw head power by de-stroking the main pump of the hydraulic system so that demands on the engine from other hydraulic systems do not override the available engine power, and thus do not compromise saw head power and the felling operation, which is the chief purpose of the machine. However, prioritizing saw operation over the main hydraulic system has obvious disadvantages, including slowed or impaired operation of one or more other hydraulic systems of the machine. Even so, such a control scheme often provides slow saw recovery relative to the operational times of other hydraulic systems of the machine, particularly in modern machines using electronic control of the hydraulic system which map the hydraulic power demands to the power and torque characteristics of the particular engine in the machine.